Semiconductor laser diodes have numerous advantages. They are small in that the widths of their active regions are typically submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (˜800 nm) which is near the absorption spectrum of common solid state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the junction increases.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a specific output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes that are used to pump a Nd:YAG rod or slab should emit radiation at about 808 nm since this is the wavelength at which optimum energy absorption exists in the Nd:YAG. But, for every 3.5° C. to 4.0° C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Accordingly, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by laser diodes, dissipation of the heat becomes more problematic since it becomes necessary to densely pack a plurality of individual diodes into arrays which generate the required amounts of input power for the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes decreases. This aggravates the problem of heat extraction from the arrays of individual diodes.
To remove heat from the laser diodes, some laser diode array packages have used beryllium oxide which has a relatively high thermal conductivity while being electrically insulative. One known commercially available package which attempts to resolve these thermally-related problems by use of beryllium oxide is produced by Laser Diode Array Inc. of Auburn, N.Y. This package generally includes a beryllium oxide structure into which a plurality of grooves are cut, etched or sawed. A metallized layer extends from groove to groove to conduct electricity through the laser diodes that are within the grooves.
However, beryllium oxide is a hazardous material and requires additional care in handling. This is especially true when the beryllium oxide is being mechanically processed (e.g. cutting or sawing) which produces airborne particles of the beryllium oxide. Because it requires additional care in handling and shipping (e.g. additional BeO warning labels), it is relatively expensive when considering the cost of the overall laser diode array package. Additionally, once the laser diode bar is placed within the groove, its reflective surface is not accessible for cleaning after the array has been assembled. Furthermore, it is difficult to test an individual laser diode bar before it is placed in the grooves. Thus, a laser diode bar lacking the desired operational characteristics for a specific array must often be removed from a groove after it has been installed.
A need exists for a thermally efficient laser diode package which is easy to assemble and test, and which preferably lacks the hazardous beryllium oxide.